Essay Example on Oscillating frequency and modulating the drain current of MOS









This project exploits the concept of change in the oscillating frequency and modulating the drain current of MOS integrated cantilever structure caused by the binding of an analyte to the cantilever This requires design and development of high sensitivity high resolution innovative functionalized cantilever which is a real technological challenge The toughest landmark of the project will be the integration of MEMS structure with CMOS process leading to the development high resolution high sensitivity device for early diagnostic of deadly diseases This research proposal is a step towards conceptualizing the design fabrication and testing of MEMS integrated CMOS system on chip Soc capable of performing real time bio molecular analysis and early diagnostic of deadly diseases In the present research project we plan is to design and develop a high resolution high sensitivity MEMS and CMOS integrated system on chip These CMOS based MEMS micro array will be capable of bio molecular sensing in attogram range for an early diagnostics of toxins hormones proteins bacteria and DNA strands etc The SoC planned to be developed in this project will also be capable of performing the real time analysis of reaction of an antibody to its antigen 

Out of several emerging techniques for sensing diagnostics and measuring the bio molecular concentration the micro cantilever detection paradigm has attracted considerable attention in recent years Ion Sensitive Field Effect Transistor ISFET and frequency shift of micro cantilever are used to detect the label free biomolecules However life time of the ISFET is limited due to fast degradation of gate oxide in the presence of the bio molecules whereas the micro cantilever requires bulky circuits to detect the electrical signal that consumes high power An MOSFET embedded micro cantilever for measuring deflection caused by the binding of bio molecules has been proposed by Shekhawat et al This approach relies on the stress induced in the channel region of the MOSFET embedded cantilever due to bending of the cantilever The stress caused by the binding of bio molecules to immobilized cantilever modulates the output drain current leading to detection of diverse bio molecular or chemical recognition events or measuring the activity of a biochemical in an organism However such a scheme has a severe drawback of poor sensitivity and resolution and attogram range resolution cannot be achieved A better approach of modulating the channel of MOSFET and its output drain current could be changing the gate voltage by using the sensitized layer coated cantilever to sense the bio molecule or bio chemical reaction which is referred as static property In addition it is proposed to combine the dynamic property obtained from the change in the frequency of the oscillating cantilever along with the static content providing another degree of freedom in the sensing technique In this process the gate of the MOSFET and the micro cantilever are to be integrated with the chosen chemical sensing layer To reduce the power consumption and enhance the life time of sensing devices an integrated approach of micro cantilever embedded 

MOSFET is proposed to be developed in the present research project The static property accounting for the change in operating point and drain current of the MOS structure and dynamic property derived from the change in oscillating frequency of the cantilever will be used to obtain high resolution and sensitivity Operation of the proposed Project Here we propose the use of 2D microcantilever arrays with geometrically configured metal oxide semiconductor field effect transistors MOSFETs embedded in the high stress region of the microcantilevers to measure deflections induced by biomolecular binding The two arrays of MosFET MEMS each have been considered one with receptor layer and the other one without receptor layer to be used as a reference When the target molecules attach to their functionalized surface the surface stress distribution on the surface is changed causing deflections in the cantilever During adsorption of target molecules onto the functionalized cantilever surface biochemical reactions occur which reduces the free energy of the cantilever surface The reduction in free energy of one side of cantilever is balanced by increase in strain energy of the other side producing deflection in the cantilever 

The deflections may be upward or downward depending on the type of molecules involved and are linearly proportional to the target analyte solution concentration It means that higher deflections manifest higher sensitivity in the cantilever biosensor When a microcantilever bends as a result of adsorption induced surface stress the modulation of the channel current underneath the gate region results from altered channel mobility of the transistor due to increased channel resistance As fixed biased voltages are applied on the gate and source drain region of the transistor any change in channel mobility will result in change in the drain current of the transistor Because MOSFETs and contact pads are passivated with a thin coating of silicon nitride they are inherently protected from any environmental influence thus their performance is not compromised by contact with gases or corrosive liquids such as saline solution The MOSFET embedded microcantilever detection approach is illustrated in Fig 1 The output of these two MOSFET are passed through a differential stage Using the differential stage leads to the cancellation of noise and hence improves SNR The frequency shift can be obtained through correlating the output of the differential stage corresponding to sensing array with that of the reference one This output of differential unit is then passed to microcontroller which has two functionalities Firstly it will Display the result on screen so user can understand the result Secondly it will connect to Lab Hospital server so a detailed report can be generated

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